Manufacture of finely divided silica



Patented Dec. 26, 1950 MANUFACTURE or FINELY mvnmo SILICA Donald B. Broughton, Boston, Mass assignor to Godfrey L. Cabot, Inc., Boston, Mass., a corporation of Massachusetts No Drawing. Application September 3, 1946,

Serial No. 694,680

1 Claim. 1

This invention comprises a new and improved process of making silica in finely divided form, or of so-called pigment grade, suitable for use as a reinforcing agent in compounding rubber or as a pigment. The process of my invention is characterized by the step of hydrolyzing silicon tetrafiuoride vapor under specially advantageous conditions which will hereinafter be disclosed.

Silica, of pigment grade or fineness has been made heretofore by burning organic compounds containing silicon, such as ethyl-ortho silicate, and then collecting the products of combustion, but these prior processes have not been entirely satisfactory from the standpoint of economy and in commercial practice have required careful technical supervision and control. Moreover, the product must be subjected to various troublesome separating or purifying steps in conditioning it for use. It has also been known that silicon tetrafiuoride vapor might be hydrolyzed by bubbling it through a body of water but so far as known this has never been carried out on a commercial scale nor has the resulting product been suitable for use in compounding rubber.

The present invention is based on my discovery that by reacting silicon tetrafiuoride in a dry state with steam, silica is produced in a totally unexpected and unpredictable degree of fineness, that is to say, as an impalpable amorphus white powder having a high specific surface area and having valuable characteristics as a reinforcing agent in natural and synthetic rubber compounds.

Silicon tetrafiuoride for use in the process of my invention may be prepared from very inexpensive and readily available raw materials, namely, fluorite, sand and sulfuric acid. Fluorite, also called fiuorspar, is found in massive and granular forms and is mined in Illinois and'Kentucky as wel1 as in Europe. Fluorite and sand react in a concentrated sulfuric acid solution according to the known reaction:

When these materials are mixed and heated in a sulfuric acid solution of a concentration of about 70%, the tetrafiuoride (Sim) vapor comes off and is directly available for hydrolysis in accordance with the present process. Hydrolysis of tetrafiuoride in the liquid phase proceeds according to the following reaction:

If hydrolysis is carried out in the vapor phase the reaction is as follows:

The process of my invention may be carried out in a dry state by mixing silicon tetrafiuoride vapor with stream, all at a temperature above that at which condensation can take place. For example, steam may be introduced into a flowing current of SiFl. and directed with it into a bag filter. The reaction begins immediately when contact is made and is evidenced by a white mist that becomes visible in the gas current. The whole apparatus is kept above the temperature of condensation until the bag filter is substantially filled with finely divided amorphus silica. The silica may then be removed from the bag as an impalpable white powder having an initial apparent density of approximately 4 lbs. per cubic foot and a specific surface area as high as -200 square meters per gram, and the step may be repeated or duplicated as desired.

Hydrolysis in the vapor phase has the advantage over hydrolysis in the liquid phase that the silicon tetrafiuoride is converted completely to silica and that no recovery is required to get complete utilization of the silicon in the silicon tetrafiuoride.

In liquid phase hydrolysis a gel is first formed which must be rendered anhydrous by heating and the final product is crystalline, whereas the product formed by hydrolysis in the vapor phase is obtained directly as anhydrous silica without ever passing through the hydrous stage. This latter process results in a product having properties different from and superior to finely divided silica hitherto available.

In the process of my invention, hydrofluoric acid is a by-product which can readily be put to use. It can be passed through sand to produce fiuosilicic acid. Thereafter the fluosilicic acid is collected and treated with sulphuric acid to make silicon tetrafiuoride which is in turn hydrolyzed by the process of my invention to produce silica of rubber pigment quality.

In another aspect, therefore, my invention comprises a process in which pigment grade silica is prepared from the end products of hydrolysis of tetrafiuoride vapor, that is to say, from fiuosilicic acid broken down into tetrafiuoride combined with more tetrafiuoride formed by passin hydrofluoric acid through sand together with the tetrafiuoride already available.

Having thus disclosed my invention and described in detail an illustrative manner in which it may be carried out I claim as new and desire to secure by Letters Patent:

The process of producing an amorphous and finely divided silica which comprises reacting aasaoao mm and H10 in vapor phase and at a tempera- "nre which prevents the formation of moisture, collecting said silica after its formation separate izom the reaction and maintaining said collected mica at a sumciently high temperature to prevent the formation oi moisture therein, maintaining said temperature until the HP has substantially separated therefrom, and then lowerin: the temperature of the collected silica.

DONALD B. BROUGHTON.

REFERENCES CITED The following references are of record in the file of this patent:

4 UNITED STATES PATENTS Number Name Date 2,428,178 Relk Sept. 30, 1947 OTHER REFERENCES Ann. Chim. Phys vol. 69 (1), p. 204

"Dictionary of Applied Chemistry," by Thorpe, vol; 4 (1918). p. 6'14.

Inorganic and Theoretical Chemistry by Mellor, vol. 6. Longmana, Green and 00., N. Y. (1925). PP. 935, 937, 942 and 943. 

